Originally posted by roughrider
A plot device, as a character, is someone who just shows up to lose a fight, so the other, more popular hero will look good - no matter how powerful said plot device could be. Like the vast majority of times Gladiator appears, losing fights he shouldn't, just so the more mainstream hero looks better. Gladiator is psycho-powerful, but is a fringe character, so apparently it makes it OK.
That could be considered an example of a plot device. I was referring to Tyrannus as a plot device because he was an apparently unstoppable and extra-badassed adversary who appears seemingly out of nowhere just to create a dramatic tension for the MTU series. He's another "Super-Skrull" with armor that feeds off of 31 flavors of energy (including gamma rays), is resistant to everything, regenerates like nobody's business, etc. He's clearly more powerful than just about everyone, and he sticks out like a sore thumb in the continuity. He's a walking, talking plot point...sort of like the Anti-Monitor for CoIE, or the Beyonder for Secret Wars (though Beyonder didn't really walk around). He is a catalyst for the story personified, and thus, a plot device.